(a) Field of the Invention
The present invention deals with the protection of current transformers by shunting their secondary windings and, more particularly, to a protection circuit for guarding the secondary windings of current transformers against overvoltages appearing across the terminals of such secondary winding, said protection system being connected in shunt with the secondary winding of a current transformer.
(b) Description of Prior Art
The continuously growing demand for electrical energy has given rise to the design and use of current transformers which are overdimensioned and of a high turns ratio. However, in operation, such transformers present a definite risk, particularly when a circuit connected to the secondary winding accidentally opens, causing large overvoltages across the terminals of the secondary. Although the current transformers now available have been designed and dimensioned so as not to reach magnetic-core saturation in the worst cases of transient current waves, since in practice the saturation voltage cannot be reached, it nevertheless occurs that the opening of a circuit on the secondary side produces an electric arc at a point in the external circuit located between the transformer secondary winding and the opening point, where the dielectric strength of the circuit is weakest. That arc is a powerful one, since its voltage is maintained constant through each application of the alternating current. This presents high risks to maintenance or repair personnel working on such secondary circuits and, furthermore, it may heavily damage the equipment related to said secondary circuit.
A device currently exists which is capable of preventing to some extent the above-mentioned risks by reducing to a safe level the overvoltages appearing across the secondary terminals of current transformers. Said device incorporates a high-power varistor mounted on a copper plate connected to a transformer secondary winding, the temperature rise of which is controlled by a thermostat which short-circuits the secondary winding to limit the varistor to a maximum temperature when the permanent fault occurs in the secondary circuit. However, this device has the disadvantage of being cumbersome, heavy and relatively costly and it generates a substantial amount of heat, therefore requiring additional cooling systems. Such drawbacks have heavily restricted the use of this type of apparatus with current transformers.
A feature of the present invention is that it is light and compact, and costs substantially less than known devices. Moreover, a substantial advantage of the protection system of the present invention is that it does not generate any heat under normal operating conditions of current transformers as well as during the occurrence of faults. Therefore, the protection system of the present invention avoids the inconveniences and drawbacks of the devices known to data, while keeping the voltages present in the secondary circuit of current transformers at a safe level.